Coarse tuning for fractional-N synthesizers having reduced period comparison error

ABSTRACT

An improved coarse tuning process for fractional-N frequency synthesizers is provided. In general, a coarse tuning circuit controls a phase lock loop (PLL) of a frequency synthesizer. During coarse tuning, a reference signal used to control an output frequency of the PLL is provided to the coarse tuning circuitry and is divided by a factor M to provide a divided reference signal. A controllable oscillator (CO) output signal from a CO in the PLL is divided by an N divider in the PLL to provide a divided CO signal. The periods or, equivalently, frequencies of the divided CO signal and the divided reference signal are compared, and the result is used to select an appropriate tuning curve for the CO. In order to reduce a period comparison error, synchronization circuitry operates to synchronize the N divider of the PLL and an M divider of the coarse tuning circuit.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of provisional patent application Ser. No. 60/569,900, filed May 11, 2004, the disclosure of which is hereby incorporated by reference in its entirety.

This U.S. patent application is related to the following concurrently filed U.S. patent application: U.S. patent application Ser. No. 10/901,669, COARSE TUNING FOR FRACTIONAL-N SYNTHESIZERS by Humphreys et al., which is hereby incorporated by reference in its entirety.

FIELD OF THE INVENTION

The present invention relates to a fractional-N frequency synthesizer, and more particularly to improved coarse tuning for a fractional-N frequency synthesizer.

BACKGROUND OF THE INVENTION

Frequency synthesizers are used in many systems, including microprocessors and radio frequency (RF) communication systems. Frequency synthesizers of the phase locked loop (PLL) type contain controlled oscillators (CO) that are typically current controlled oscillators (ICO) or voltage controlled oscillators (VCO). The output of the CO is often used as an injection signal for an RF mixer or in a microprocessor clock system. The frequency synthesizer controls the CO such that its frequency or period is approximately equal to that of a stable frequency reference multiplied by a predetermined ratio. In many applications, there is a need to compensate for process and environmental variations that affect synthesizer performance. The prior art contains many systems, known as coarse tuning systems, that compensate for variations in the CO's frequency tuning range or center frequency by performing a coarse tuning of the tunable elements in the CO. These systems have been developed for integer-N synthesizers where the divided CO signal used for feedback to the phase detector has a steady period when the CO's output frequency is not changing.

The use of fractional-N synthesizers provides reduced lock times for the synthesizer's phase lock loop (PLL) and improves noise performance, but introduces significant jitter on the divided CO signal. Even with a constant CO output frequency, the divider modulus, and therefore the period of the divided CO signal, is varied from one output clock cycle to the next to provide a desired average fractional modulus over a period of time. The resulting jitter on the divided CO signal significantly reduces the accuracy of existing techniques for calibrating and tuning the CO. Accordingly, there is a need for a technique to reduce the impact of jitter on the divided CO signal and provide accurate calibration or coarse tuning of a CO in a fractional-N synthesizer in a cost-effective and efficient manner.

SUMMARY OF THE INVENTION

The present invention provides an improved coarse tuning process for fractional-N frequency synthesizers is provided. In general, a coarse tuning circuit controls a phase lock loop (PLL) of a frequency synthesizer. During coarse tuning, a reference signal used to control an output frequency of the PLL is provided to the coarse tuning circuitry and is divided by a factor M to provide a divided reference signal. A controllable oscillator (CO) output signal from a CO in the PLL is divided by an N divider in the PLL to provide a divided CO signal. The periods or, equivalently, frequencies of the divided CO signal and the divided reference signal are compared, and the result is used to select an appropriate tuning curve for the CO. Synchronization circuitry operates to synchronize the N divider of the PLL and an M divider of the coarse tuning circuit. The synchronization circuitry is clocked by a clock signal having a frequency significantly greater than the frequency of the reference signal, thereby reducing error associated with the comparison of the frequency of the divided reference signal and a frequency of the divided CO signal.

In one embodiment, the coarse tuning circuit controls the PLL such that the PLL operates in an integer division mode during coarse tuning and switches to a fractional-N division mode during normal operation, thereby eliminating jitter during coarse tuning. The coarse tuning circuit includes divide value generation circuitry that provides a fractional-N sequence to an N divider during fractional-N operation and an integer divide value to the N divider of the PLL during coarse tuning.

Those skilled in the art will appreciate the scope of the present invention and realize additional aspects thereof after reading the following detailed description of the preferred embodiments in association with the accompanying drawing figures.

BRIEF DESCRIPTION OF THE DRAWING FIGURES

The accompanying drawing figures incorporated in and forming a part of this specification illustrate several aspects of the invention, and together with the description serve to explain the principles of the invention.

FIG. 1 illustrates a fractional-N frequency synthesizer according to one embodiment of the present invention;

FIG. 2A illustrates a resonant tank of the voltage controlled oscillator of the fractional-N synthesizer of FIG. 1 according to one embodiment of the present invention;

FIG. 2B is a more detailed illustration of the resonant tank of FIG. 2A;

FIG. 3 illustrates VCO output versus tuning voltage according to one embodiment of the present invention;

FIG. 4 is a illustrates the general process of coarse tuning according to one embodiment of the present invention;

FIG. 5 illustrates the divide value generation circuitry according to one embodiment of the present invention;

FIG. 6 illustrates the N divider circuit according to one embodiment of the present invention;

FIG. 7 illustrates a timing diagram of the frequency synthesizer according to one embodiment of the present invention; and

FIG. 8 illustrates a method of coarse tuning a fractional-N frequency synthesizer according to one embodiment of the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The embodiments set forth below represent the necessary information to enable those skilled in the art to practice the invention and illustrate the best mode of practicing the invention. Upon reading the following description in light of the accompanying drawing figures, those skilled in the art will understand the concepts of the invention and will recognize applications of these concepts not particularly addressed herein. It should be understood that these concepts and applications fall within the scope of the disclosure and the accompanying claims.

FIG. 1 illustrates a fractional-N frequency synthesizer 10 according to the present invention. During coarse tuning, the frequency synthesizer 10 operates in an integer divide mode, thereby eliminating all frequency error due to jitter. Once coarse tuning is performed, the frequency synthesizer switches to normal operation in a fractional-N division mode and locks to the desired frequency.

With reference to FIG. 1, the fractional-N frequency synthesizer 10 includes a fractional-N phase lock loop (PLL) 12 and coarse tuning circuitry 14. The PLL 12 generates a desired frequency for the output signal, F_(VCO), 16, of a voltage controlled oscillator (VCO) 18. In traditional fashion, the output signal F_(VCO) 16 is also provided to divider circuitry 20 to divide the output signal F_(VCO) by a factor N to produce a divided VCO signal F_(V), which is fed to one of two inputs of a phase detector 22.

A reference frequency, F_(REF), 24, is divided by a factor R by divider circuitry 26 to produce a reference signal, F_(R), which is provided to the other input of the phase detector 22. The N and R factors of the N divider 20 and the divider circuitry 26, respectively, are selected so that the frequencies of the reference signal, F_(R), and the divided VCO signal, F_(V) are equal when the desired output signal, F_(VCO), 16, is at a desired frequency. The phase detector 22 compares the relative phases of the reference signal, F_(R), and the divided VCO signal, F_(V), and provides an output relative to the difference in phase to control the VCO 18. The output of the phase detector 22 is preferably provided by a charge pump and filtered by a loop filter 28 to provide a stable voltage for a VCO fine tuning signal, V_(T), to adjust the desired output frequency F_(VCO) of the VCO 18 based on the difference in phase of the reference signal, F_(R), and the divided VCO signal, F_(V).

The coarse tuning circuitry 14 of the present invention effectively calibrates the fine tuning frequency range of the VCO 18 by selecting the appropriate tuning curve. During coarse tuning, the VCO fine tuning signal V_(T) is forced to a desired initialization value (voltage or current) under the control of tuning logic 30. Using a switch 32, the initialization value, V_(INIT), is coupled to the output of the loop filter 28 to force the VCO fine tuning signal V_(T) to the initialization value, V_(INIT). Optionally, the initialization value, V_(INIT), may be coupled to the input of the loop filter 28. The switch 32 may take many forms, including a transistor biased to provide a switching function. The initialization value, V_(INIT), may be provided using a variety of techniques. In one embodiment, control logic 34 provides a digital value to a digital-to-analog converter (DAC) 36, which provides a corresponding analog voltage for the initialization value, V_(INIT). The tuning logic 30 will typically provide a loop filter control signal, LF_(CON), to cause switch 32 to apply the initialization voltage, V_(INIT), to the output of the loop filter 28.

During coarse tuning, the tuning logic 30 also switches divide value generation circuitry 38 into an integer mode. In this embodiment, the divide value generation circuitry 38 is switched to the integer mode by the assertion of the loop filter control signal, LF_(CON). According to the present invention, when the loop filter control signal, LF_(CON), is asserted, the divide value generation circuitry 38 generates a divide value 40 that is an integer equal to the integer component of the desired N value multiplied by the modulus M of M divider circuit 42. The desired N value is defined by the combination of an integer value, N_(INT), 44, and a fractional component. Typically, a number, NUM 46, represents a numerator of the fractional component. Accordingly, during coarse tuning, the divided VCO output signal is the VCO output signal F_(VCO) divided by N*M.

The M divider circuit 42 divides the reference signal, F_(R), by a factor of M to create a divided reference signal, F_(RM). In one embodiment, the divider circuit 42 provides a modulus of 64 (M=64). Further, the modulus M of the divider circuit 42 is preferably a power of two such as but not limited to 2, 8, 16, 32, 64, 128 . . . , as is discussed in more detail below. The tuning logic 30 processes the divided reference signal F_(RM) and the divided VCO signal, F_(V), to provide a VCO control signal VTC_(CON), which controls calibration of the VCO 18. In the preferred embodiment, the tuning logic 30 is a state machine configured to compare the periods of the divided VCO signal F_(V) and divided reference signal F_(RM) and provide a pulse for the control signal VTC_(CON) to control a coarse tune counter 48. The output of the coarse tune counter 48 provides select signal VTC_(SEL) to select a capacitance value of the VCO's resonant circuit. Alternatively, the tuning logic 30 may generate the VCO select signal VTC_(SEL) directly, for example, as the output of a state machine that implements a binary search. Based on the comparison of the divided VCO signal F_(V) and the divided reference signal F_(RM), the tuning logic 30 calibrates the VCO 18.

Once coarse tuning is complete, the loop filter control signal, LF_(CON), is deasserted such that the divide value generation circuitry 38 is switched to fractional-N mode and the frequency synthesizer 10 operates as a fractional-N frequency synthesizer.

With reference to FIG. 2A, one embodiment of the VCO 18 has a tank circuit 50 including parallel inductance 52 (L), fixed or parasitic capacitance 54 (C_(P)), variable capacitance 56 (C_(V)), and switched capacitance 58 (C_(SW)). The resonant frequency of the tank circuit is defined by:

$f = {\frac{1}{2\pi\sqrt{L\left( {C_{p} + C_{sw} + C_{v}} \right)}}.}$

The variable capacitance 56 may be a varactor or other continuously variable capacitance. The variable capacitance 56 is typically used for fine tuning under the control of the fine tuning signal V_(T) provided by the PLL 12.

As shown in FIG. 2B, one embodiment of the switched capacitance 58 is a network of capacitors 60, 62, 64, which can be selectively switched into or out of the VCO tank circuit 50 via switches 66, 68, 70, respectively, for coarse tuning. The switches 66, 68, 70 can take many forms, including transistors biased to operate as switches. The capacitors 60, 62, 64 are preferably binary weighted and switched into the VCO tank circuit 50 under the control of the VCO tank capacitance select signal VTC_(SEL) from the coarse tune counter 48. In one embodiment, the VCO tank capacitance select signal VTC_(SEL) is a three bit digital signal, where the bits correspond to the respective capacitance values C, 2C, 4C for the capacitors 60, 62, 64, respectively. Accordingly, VTC signal values 000 (binary) through 111 will correlate to capacitance values 0 through 7 for the switched capacitance 58 of the VCO tank circuit 50. Alternatively, the capacitors may be unit weighted, for example to allow an interdigitated layout for better matching, as is well known in the art. In this case, each bit of the VTC signal is connected in parallel to the switches 66, 68, and 70 controlling the capacitors 60, 62, and 64 that comprise the corresponding capacitance for that bit. For example, the least significant bit (bit 0) of the VTC signal is coupled to the switch 66 that controls the capacitor 60 having a weight C. Bit 1 of the VTC signal is coupled to switch 68 that controls the capacitor 62 of weight 2C. Alternatively, the capacitor 62 may be two capacitors of weight C such that the combined capacitance is 2C, and the switch 68 may be replaced by two switches each controlled by bit 1. Similarly, bit 2 of the VTC signal is coupled to the switch 70 that control the capacitor 64 having a weight 4C. This arrangement provides balanced parasitic capacitances that scale proportionally with the tuning curve selection, yielding evenly spaced tuning curves.

The operating characteristics for an exemplary VCO 18 having the above-configured VCO tank circuit 50 are shown in FIG. 3. Since the switched capacitance 58 can be configured to have one of eight capacitance values, the VCO 18 will have eight tuning curves corresponding to the respective capacitance values. The bottom tuning curve corresponds to having all three capacitors 60, 62, 64 switched into the network when the select signal VTC_(SEL) is 111. The top tuning curve corresponds to having all three capacitors 60, 62, 64 switched out of the network when the select signal VTC_(SEL) is 000. Depending on the fine tuning signal voltage, V_(T), there are multiple tuning curves capable of providing a selected frequency. For best noise performance and more consistent tuning gain for the VCO 18, the voltage of the fine tuning signal, V_(T), should be kept within an acceptable range. Typically, the Q of the varactor 56 tends to fall off at higher capacitance and higher tuning voltage values. For this example, assume the preferred operating range for the VCO 18 corresponds to a fine tuning signal voltage V_(T) between 0.4 and 1.6 volts.

FIG. 4 generally illustrates the operation of the coarse tuning circuitry 14 according to the present invention with respect to exemplary operating characteristics of the VCO 18. As an example, the desired tuning curve of the VCO 18 may be defined as the lowest frequency tuning curve on which the VCO signal, F_(VCO), is higher than a desired lock frequency, F_(LOCK). During coarse tuning, the tuning voltage, V_(T), of the VCO 18 is forced to the initialization value, V_(INIT). Based on the comparison of the divided VCO signal, F_(V), and the divided reference signal, F_(RM), the tuning logic 30 steps through the tuning curves until the desired tuning curve is selected. Once the tuning curve is selected, the tuning logic de-asserts the loop filter control signal, LF_(CON), such that the frequency synthesizer 10 fine tunes the output of the VCO 18 to the lock frequency, F_(LOCK).

FIG. 5 illustrates one embodiment of the divide value generation circuitry 38. In general, the divide value 40 from the divide value generation circuitry 38 is an output of decision circuitry 76, N_(INST). In this embodiment, the decision circuitry 76 is controlled by the loop filter control signal, LF_(CON). During coarse tuning, the loop filter control signal, LF_(CON), is asserted and the divide value, N_(INST), is an integer component of the product of a desired N value and the modulus M. During fractional-N operation, the loop filter control signal, LF_(CON), is de-asserted and the divide value, N_(INST), is a fractional sequence that is the sum of the integer, N_(INT), and a sequence (SEQ). The sequence (SEQ) is a sequence of integer values having an average value equal to the desired fraction.

At a first input, the decision circuitry 76 receives the product of the desired N value and the modulus M of the M divider 42. The desired N value is the combination of the integer N_(INT) and the numerator of the fractional component, NUM. In this embodiment, the modulus M of the divider 42 is a power of two (2, 4, 8, 16, 32, 64, . . . ). Thus, the multiplication of the modulus M and the desired N value is preferably performed by left shifting the bits of the desired N value by log₂(M) bits. However, other methods of multiplication may be used, and modulus M may be a number that is not a power of two.

At a second input, the decision circuitry 76 receives the sum of N_(INT) and the sequence (SEQ) from summation circuitry 78. The sequence (SEQ) of integer values is generated by a fractional sequence generator 80 based on the number, NUM, representing a numerator of the fractional component. In one embodiment, the fractional sequence generator 80 employs a third order MASH style delta-sigma modulator.

FIG. 6 illustrates one embodiment of the N divider circuit 20. In general, A,B logic 82 generates count values A and B from the divide value N_(INST) from the divide value generation circuitry 38 using the equation: N _(INST)=(P×A)+B, where P is the first modulus of a dual modulus prescaler 88. The prescaler 88 operates to divide the VCO output signal F_(VCO) by either P or P+1 based on a modulus control signal MC from the B counter 86. The A counter 84 divides the output signal P_(OUT) from the prescaler 88 by the A count value, thereby providing the divided VCO signal F_(V).

The N divider circuit 20 also includes a synchronizer 90 including first and second flip flops 92 and 94. The synchronizer 90 operates to re-time the reset signal RST from the reference signal F_(R) domain to the prescaler output signal P_(OUT) domain. The benefit of the synchronizer 90 is illustrated in FIG. 7.

FIG. 7 illustrates a timing diagram for the frequency synthesizer 10 of the present invention during coarse tuning. After M cycles of the F_(R) signal and N_(INST)*M cycles of the VCO signal, F_(VCO), the signals F_(RM) and F_(V) are asserted. The time period A is the time between when the M divider 42 and the N divider 20 may start. This period is a significant source of error in the comparison of the period of the divided reference signal F_(RM) and the period of the divided VCO signal F_(V) during coarse tuning. By clocking the synchronizer 90 using the prescaler output POUT, the period A is minimized and the period comparison error is reduced. In another embodiment, the flip flops 92 and 94 of the synchronizer 90 are capable of being clocked by the VCO output signal F_(VCO), and the synchronizer is clocked by the VCO output signal F_(VCO), thereby further reducing the time period A and minimizing the period comparison error. By minimizing the period comparison error, the overall frequency error of the frequency synthesizer 10 is also reduced.

In operation, a coarse tuning process is carried out each time the PLL 12 is enabled. The tuning process selects an appropriate capacitance setting for the switched capacitance 58 within a period of time sufficiently short that it does not significantly impact the required warm-up time for the PLL 12. In general, the process operates by dividing the VCO output signal F_(VCO) by the integer component of the product of the desired N value and the modulus M of the M divider 42. The periods of the divided VCO signal F_(V) and the divided reference signal F_(RM) are compared and an appropriate capacitance is selected for the switched capacitance 58 to select the tuning curve of the VCO's output signal F_(VCO) 16. In one embodiment, the process repeats for each tuning curve by progressively incrementing or decrementing different capacitance 58 values until an acceptable tuning curve is selected. A binary search algorithm may be implemented to reduce the number of comparison steps, at the expense of increased complexity in the tuning logic 30. Further details for the tuning process according to one embodiment of the present invention are provided in association with the flow diagram of FIG. 8.

The coarse tuning process begins (step 100) by forcing the tuning voltage of the VCO fine tuning signal V_(T) to the initialization voltage V_(INIT) (step 102). For the illustrated example, the VCO fine tuning signal V_(T) is forced to 0.4 volts. Assume the control logic 34 sends a digital value to the DAC 36 to set the initialization voltage, V_(INIT), to 0.4 volts. The tuning logic 30 will then activate the loop filter control signal LF_(CON) to close switch 32 and apply the initialization voltage, V_(INIT), to the input of the VCO 18. Alternatively, the initialization voltage V_(INIT) may be applied to the loop filter 28, which is typically a low pass filter, thereby forcing the VCO fine tuning signal V_(T) to 0.4 volts.

The divide value generation circuitry 38 is switched into integer mode (step 104). In integer mode, the divide value of the N divider circuit 20 is set to the integer component of the product of the desired N value and the modulus M of the M divider 42. As discussed above, the desired N value is determined based on the combination of the integer N_(INT) and the numerator NUM of the fractional component.

Next, the capacitance of the tank circuit 50 for the VCO 18 is set to a tuning curve corresponding to a minimum frequency range (step 106). Accordingly, the coarse tune counter 48 generates a select signal VTC_(SEL) to switch each of the capacitors 60, 62, 64 into the switched capacitance 58 to set the VCO 18 to operate at the minimum frequency range. In this example, the select signal VTC_(SEL) is initialized to 111. In alternative embodiments, the VCO 18 may be initialized for the maximum frequency range.

The tuning logic 30 then resets the dividers 20 and 42 by asserting the reset signal RST (step 108), and compares the frequencies, or periods, of the divided reference signal F_(RM) and the divided VCO signal F_(V) (step 110).

In the preferred embodiment, the divider 42 receives the reference signal F_(R) and provides an output pulse every M cycles to generate the divided reference signal F_(RM). Similarly, the divider 20 receives the VCO signal F_(VCO) and provides an output pulse every N*M cycles to generate the divided VCO signal F_(V). The tuning logic 30 receives both the divided reference signal F_(RM) and the divided VCO signal F_(V). Preferably, the tuning logic 30 resets each of the divider circuits 20 and 42 with the RST signal (step 108), and then senses whether the first output pulse of the divided VCO signal F_(V) occurs before the first output pulse of the divided reference signal F_(RM), indicating that the frequency of the divided VCO signal F_(V) is higher than that of the divided reference signal F_(RM). Because the frequency of the divided reference signal F_(RM) and the frequency of divided VCO signal F_(V) are simply equal to the frequency of the reference signal F_(R) divided by M and frequency of the VCO signal F_(VCO) divided by N*M, respectively, this comparison also indicates whether the frequency of the VCO is higher than the desired operating frequency.

In the preferred embodiment, the desired tuning curve is the lowest frequency tuning curve on which the frequency of the output signal F_(VCO) at initialization voltage V_(INIT), is higher than that desired. By comparing the respective periods, if the frequency of the divided VCO signal F_(V) is determined to be less than that of the divided reference F_(RM) signal (i.e. the period for F_(V) is greater than F_(RM)) (step 112), the tuning logic 30 preferably provides a control signal VTC_(CON) to decrement the coarse tune counter 48. Decrementing the coarse tune counter 48 results in a VCO select signal VTC_(SEL) decrementing the capacitance value of the switched capacitance 58 to select the next higher tuning curve, which corresponds to the next higher frequency range (step 114).

The periods for the divided VCO signal F_(V) and the divided reference signal F_(RM) are again compared, and the switched capacitance 58 is decremented until the frequency of the divided VCO signal F_(V) is greater than the frequency of the divided reference signal F_(RM) (step 112). Once the frequency of the divided VCO signal F_(V) is greater than the frequency of the divided reference signal F_(RM), the tuning logic 30 releases the initialization voltage V_(INIT) from the input of the VCO 18 and controls the divide value generation circuitry 38 such that it provides the fractional sequence (N_(INT)+SEQ) by deactivating the LF_(CON) signal, thereby switching to fractional-N mode and allowing the PLL 12 to lock (step 116) and the process ends (step 118). The tuning logic 30 will maintain the switched capacitance 58 at the appropriate capacitance via the VCO select signal VTC_(SEL) to ensure that the VCO 18 operates at the appropriate tuning curve.

Alternatively, the VCO 18 may be initialized at the maximum frequency tuning curve wherein the select signal VTC_(SEL) is 000. During coarse tuning, the VCO fine tuning signal V_(T) is forced to 1.6 volts, the divider value 30 is forced to the integer component of the desired N value and the modulus M of the M divider 42, and the switched capacitance 58 is incremented until the frequency of the divided VCO signal F_(V) is less than the frequency of the divided reference signal F_(RM).

The maximum duration of the coarse tuning process is approximately the period of the divided reference frequency, F_(RM) (M*R/F_(REF)), times the number of capacitance settings for the switched capacitance 58 of the VCO tank circuit 54. For a 26 MHz reference frequency F_(REF), an R value of 1, an M value of 64, and eight capacitance settings, the coarse tuning operation would take at most 20 microseconds before the phase locking in the PLL 12 can begin. Accordingly, the impact of coarse tuning on locking time of the PLL 12 is minimal. Alternatively, a binary search algorithm may be employed, in which only one frequency comparison is required for each bit of the VCO control signal VTC_(CON). In this example, only three comparisons would be needed instead of eight, further reducing the duration of the coarse tuning operation.

Preferably, the invention is implemented in a manner minimizing the number of interconnections between the coarse tuning circuitry 14 and the PLL 12, especially in embodiments where the two circuits are on separate packages or semiconductor die. For example, the coarse tune counter 48 and VCO 18 may be placed on a common die, and the coarse tune counter 48 will only require one input to receive the VCO control signal VTC_(CON) to control the value of the switched capacitance 58 of the VCO tank circuit 50. This arrangement reduces the number of pins required on the associated semiconductor packages, or the number of bond pads on the associated die, reducing cost and area.

It will be recognized that the above discussion applies equally well to other types of oscillators that may be used with fractional-N frequency synthesizers. For example, ring oscillators can be constructed with coarse and fine tuning control signals provided by switched capacitive loads or switched transistor amplifier stages, and supply voltage or current tuning, respectively. Accordingly, elements, such as gain stages or impedance devices, may be selectively switched into or out of the current or voltage controlled oscillator circuitry to select a tuning curve. Further, the counters described herein may be implemented using registers to hold value, or state machines. As such, a counter is deemed to cover and include these configurations.

Additional detail pertaining to ring oscillators is found in, “An all-digital phase-locked loop with 50-cycle lock time suitable for high-performance microprocessors,” by J. Dunning et al., IEEE Journal of Solid-State Circuits, Volume 30, Issue 4, April 1995, pages 412–422.

Those skilled in the art will recognize improvements and modifications to the preferred embodiments of the present invention. All such improvements and modifications are considered within the scope of the concepts disclosed herein and the claims that follow. 

1. A fractional-N frequency synthesizer comprising: a) a phase lock loop (PLL) adapted to provide a CO output signal having a frequency determined by a reference signal and a divide value, the PLL comprising: i) a controlled oscillator (CO) adapted to provide the CO output signal; and ii) a first divider adapted to divide the CO output signal by the divide value to provide a divided CO signal; b) coarse tuning circuitry comprising a second divider adapted to divide the reference signal by a factor M to provide a divided reference signal; and c) synchronization circuitry adapted to synchronize the first and second dividers based on a clock signal derived from the CO output signal.
 2. The fractional-N frequency synthesizer of claim 1 wherein the coarse tuning logic further comprises tuning logic adapted to compare a frequency of the divided reference signal and a frequency of the divided CO signal and effectuate coarse tuning of the CO based on the comparison.
 3. The frequency synthesizer of claim 2 wherein the tuning logic is adapted to compare the frequency of the divided reference signal and the frequency of the divided CO signal by comparing the period of the divided reference frequency with the period of the divided CO frequency.
 4. The fractional-N frequency synthesizer of claim 2 wherein the tuning logic provides a reset signal to the second divider and the synchronization circuitry provides a synchronized version of the reset signal to the first divider, thereby beginning the comparison of the frequency of the divided reference signal and the frequency of the divided CO signal.
 5. The fractional-N frequency synthesizer of claim 4 wherein the first divider comprises: a) prescaling circuitry adapted to receive the CO output signal and divide the CO output signal based on a modulus control signal to provide the clock signal; and b) circuitry adapted to provide the divided CO signal and the modulus control signal based on the divide value, the clock signal, and the synchronized version of the reset signal.
 6. The fractional-N frequency synthesizer of claim 4 wherein the first divider comprises: a) prescaling circuitry adapted to receive the CO output signal and divide the CO output signal to provide the clock signal; and b) circuitry adapted to provide the divided CO signal based on the divide value, the clock signal, and the synchronized version of the reset signal.
 7. The fractional-N frequency synthesizer of claim 4 wherein the clock signal is the CO output signal and the first divider comprises: a) prescaling circuitry adapted to receive the CO output signal and divide the CO output signal based on a modulus control signal to provide a scaled CO signal; and b) circuitry adapted to provide the divided CO signal and the modulus control signal based on the divide value, the scaled CO signal, and the synchronized version of the reset signal.
 8. The fractional-N frequency synthesizer of claim 1 wherein the clock signal has a frequency greater than the frequency of the reference signal.
 9. The fractional-N frequency synthesizer of claim 1 wherein the clock signal is the CO output signal.
 10. The fractional-N frequency synthesizer of claim 2 wherein the coarse tuning circuitry further comprises divide value generation circuitry adapted to: a) provide an integer divide value to the first divider when operating in the integer division mode during coarse tuning; and b) provide a fractional sequence of divide values to the first divider when operating in the fractional-N division mode during normal operation.
 11. The fractional-N frequency synthesizer of claim 10 wherein the integer divide value is an integer component of a product of the multiplication of a desired divide value of the fractional-N synthesizer and the factor M.
 12. The fractional-N frequency synthesizer of claim 10 wherein the tuning logic is further adapted to provide a tuning curve control signal to select a tuning curve for the CO of the PLL based on the comparison of the frequency of the divided reference signal and a frequency of the divided CO signal to effect coarse tuning of the CO.
 13. The fractional-N frequency synthesizer of claim 12 wherein the tuning logic is further adapted to provide a control signal to the divide value generation circuitry, thereby controlling the mode of operation of the divide value generation circuitry.
 14. The fractional-N frequency synthesizer of claim 12 wherein the CO comprises a circuit having a selectable capacitance with a plurality of settings corresponding to a plurality of tuning curves and the tuning logic is further adapted to provide the tuning curve control signal to sequentially select the plurality of tuning curves until the frequency of the divided CO signal is within a desired proximity of the frequency of the divided reference signal.
 15. The fractional-N frequency synthesizer of claim 14 wherein the coarse tuning circuitry further comprises a coarse tune counter adapted to receive the tuning curve control signal and generate a select signal to provide to the circuit of the CO to control the selectable capacitance.
 16. The frequency synthesizer of claim 12 wherein the coarse tuning circuitry further comprises a switch for coupling an initialization signal to a fine tuning signal for the CO to override feedback from a phase detector of the PLL during coarse tuning and bias the CO to operate at a predefined control setting.
 17. The frequency synthesizer of claim 16 wherein the tuning logic is further adapted to control the switch.
 18. A method for improving coarse tuning of a fractional-N frequency synthesizer comprising: a) generating a controlled oscillator (CO) output signal based on a reference signal and a divide value; b) dividing the CO output signal by the divide value to provide a divided CO signal; c) dividing the reference signal by a factor M to provide a divided reference signal; d) synchronizing dividing steps (b) and (c) based on a clock signal derived from the CO output signal; and e) providing coarse tuning of a CO providing the CO output signal based on a comparison of the divided reference signal and the divided CO signal.
 19. The method of claim 18 wherein the clock signal has a frequency greater than the frequency of the reference signal.
 20. The method of claim 18 wherein the clock signal is the CO output signal.
 21. The method of claim 18 wherein the dividing the reference signal step is controlled by a reset signal and the synchronizing step synchronizes the dividing steps (b) and (c) based on providing a synchronized version of the reset signal for controlling the dividing the CO signal step.
 22. The method of claim 18 wherein the clock signal is a scaled version of the CO output signal.
 23. The method of claim 18 further wherein the divide value is an integer divide value when operating in an integer division mode during coarse tuning, and the divide value is a fractional sequence of divide values when operating in a fractional-N division mode during normal operation.
 24. The method of claim 23 wherein the integer divide value is an integer component of a product of the multiplication of a desired divide value of the fractional-N synthesizer and the factor M.
 25. The method of claim 18 wherein the providing coarse tuning step comprises sequentially selecting a plurality of tuning curves of a CO providing the CO output signal until the frequency of the divided CO signal is within a desired proximity of the frequency of the divided reference signal.
 26. The method of claim 18 wherein the providing coarse tuning step comprises comparing the frequency of the divided reference signal and the frequency of the divided CO signal by comparing the period of the divided reference frequency with the period of the divided CO frequency. 